38

C O R R E L A T I O N OF ROUGHOMETER AND SKID T E S T S

WITH P A V E M E N T T Y P E , DESIGN AND MIX

A. DUKE MORGAN, I f a t e r i o l s fleseorefc E n g i n e e r N o r t h C a r o l i n a S t a t e Highway and P u b l i c Works Commission

R a l e i g h , N o r t h C a r o l i n a

SYNOPSIS

Dur ing 1949 and 1950 two types o f bituminous mixtures on p l a n t - m i x pavement p ro jec t s were s tudied by the North Carolina State Highway Lab­ora tory ; namely, sand-asphalt and asphal t ic concrete

Hie purpose of the study wss to corre la te r i d i n g q u a l i t i e s on recent­l y constructed bituminous pavements w i t h mix design p o l i c i e s and con­s t r u c t i o n prac t ices .

Road roughness measurements on these two types o f pavement are repor ted . Tests were made at the speed o f 20 mph i n the d i r e c t i o n of t r a f f i c . The t r a i l e r was run i n the center o f the t r a f f i c lane being tested.

Skid resistance measurements on these two types o f pavement were made f o r the purpose o f c o r r e l s t i n g surface texture w i t h mix design i n an e f f o r t to e s t ab l i sh safe l i m i t s f o r gradation of aggregate and bitumen content.

Skid resistance messurements were made by the stoppin^g-distsnce method On l e v e l sec t ions o f pavemeAt, the car was brought to i n i t i a l u n i f o r m

speeds o f 20, 30, and 40 mph in successive tes ts , and the brskes applied i n s t a n t l y , l o c k i n g a l l f o u r wheels At the i n s t a n t o f l o c k i n g the wheels, a detonator f i r e d a chalk b u l l e t on the pavement The forward skidding, or stopping-distance, was measured at each o f the three speeds on a dry su r f ace . The pavement was then f l u s h e d w i t h water and the tests repeated

The s t s b i l i t i e s o f the sand-asphslts were measured in the labora tory by the Hubbard-Field method us ing the 2 - i n diameter mold and o f the asphal t ic concrete using the 6- in mold

In the method o f mix design, specimens were compacted under c o n t r o l l ­ed e f f o r t a t several asphal t contents us ing the proposed aggregate to determine the asphalt content which gives the most desi rable values f o r u n i t weight o f the t o t a l mix, percent voids i n t o t s l mix, percent voids f i l l e d w i t h asphalt , and s t a b i l i t y .

Resu l t s o f these s t u d i e s lead us to the f o l l o w i n g obse rva t ions

(1) S t a b i l i t i e s on sand-asphalts, using 2- in diameter specimens, ranged from a minimum of ISO l b . to a maximum of 1800 l b by the Hubbard F i e l d Method. (2) S t a b i l i t i e s on aspha l t i c concrete ranged from SOO l b . to 4500 l b s t s b i l i t y on 6 - i n . d ismeter specimens by the m o d i f i e d Hubbard F i e l d Ifethod. (3) Road roughness measurements f o r ssnd-asphalt pavements ranged from a maximum of 176 to a minimum of 54 i n . per n i .

39

(4) Road roughness measurements f o r asphal t ic concrete pavements ranged from a maximum of 117 to a minimum of 57 i n . per mi (5) Skid resistance tests on wet sand-asphalt pavements at 40 mph ranged from a skid distance o f 69 f t to a skid distance o f 126 f t (6) Skid res i s tance t e s t s on wet a spha l t i c concrete pavements at 40 mph ranged from a sk id distance o f 81 f t to a s k i d distance o f 111 f t (7) Considering only newly l a i d pavement, the lowest roughness index was obtained wi th low s t a b i l i t y sand-asphalt mixes (8) Where the preponderance of sand p a r t i c l e s occur between the No. 40 and No 200 mesh s ieves , the f r i c t i o n a l res i s tance o f the sur face i s comparatively low

THE PRESENT TREND OF B I T U M I N O U S SURFACING AND RESURFACING WORK I N

fiORTH CAROLINA

For many y e a r s t h e N o r t h C a r o l i n a S t a t e H i g h w a y and P u b l i c Works Com­m i s s i o n has been c o n s t r u c t i n g t h e more s t a n d a r d t y p e s o f b i t u m i n o u s c o n c r e t e and s a n d - a s p h a l t s u r f a c e s o f c o n v e n t i o n a l t h i c k n e s s , t h a t i s , f r o m 1 t o 3 i n . W h i l e we f e e l t h e r e have been some g r a d u a l i m p r o v e m e n t s i n t h e mix d e s i g n f o r t hese s u r f a c e s , e s s e n t i a l l y o u r d e s i g n s and a p p l i ­c a t i o n f o r such work have n o t changed g r e a t l y o v e r t h e p a s t f i f t e e n o r t w e n t y y e a r s . We h a v e , h o w e v e r , d u r i n g t h e p a s t f o u r y e a r s , c o n ­s t r u c t e d c o n s i d e r a b l e m i l e a g e o f t h i s t y p e o f s u r f a c e b o t h i n one c o u r s e and t w o c o u r s e c o n s t r u c t i o n on o l d c o n c r e t e p a v e m e n t s w h i c h w e r e i n v a r y i n g s t a g e s o f f a i l u r e , and new bases c o n s i s t i n g o f s t o n e and s o i l .

I t i s w e l l known t h a t a n g u l a r and r o u g h - t e x t u r e d a g g r e g a t e s p r o d u c e a r e l a t i v e l y h i g h d e g r e e o f s t a b i l i t y m b i t u m i n o u s m i x t u r e s . Hence , t h e r e g i o n i n c e n t r a l and w e s t e r n N o r t h C a r o l i n a w h i c h has a v a i l a b l e an abun­dance o f c r u s h e d a g g r e g a t e s , e n a b l e s t h e e n g i n e e r s t o v e r y e a s i l y s e c u r e d u r a b l e m i x t u r e s w h i c h r e s i s t s h o v i n g and c r e e p i n g . On t h e o t h e r h a n d , t h e m o s t a b u n d a n t a n d e c o n o m i c a l a g g r e g a t e s i n t h e e a s t e r n r e g i o n o f t h e s t a t e c o n s i s t o f sands and g r a v ­e l s . The sands a r e v e r y r o u n d and s m o o t h , and t h e g r a v e l s u n c r u s h e d w h i c h r e s u l t s i n r e l a t i v e l y l o w s t a b i l i t y .

S t a b i l i t y t e s t s h a v e l o n g t e e n employed by many h i g h w a y d e p a r t m e n t s as v a l u a b l e a i d s i n t h e d e s i g n o f a s p h a l t i c p a v i n g m i x t u r e s . By m a k i n g such t e s t s , i t i s p o s s i b l e t o s e l e c t b e t t e r m a t e r i a l s a n d t o f i n d t h e o p t i m u m c o m b i n a t i o n o f t h e s e l e c t e d m a t e r i a l s , so t h a t m i x t u r e s o f good q u a l i t y c a n be e x p e c t e d . Common s t a b i l i t y t e s t s f o r s u c h a p u r p o s e a re t h o s e p r o p o s e d by H u b b a r d - F i e l d , H v e e m , a n d M a r s h a l l .

I n t h e d e s i g n o f b i t u m i n o u s m i x ­t u r e s , more a t t e n t i o n s h o u l d be g i v e n t o some f o r m o f s t a b i l i t y m e a s u r e ­m e n t . The t r e n d t o w a r d c o a r s e r t y p e p l a n t m i x t u r e s s h o u l d be n o t e d , and t h a t such m i x t u r e s a r e l e s s s u s c e p ­t i b l e t o v a r i a t i o n s i n d e s i g n w i t h r e s p e c t t o s t a b i l i t y d u e t o t h e s t r u c t u r a l s t r e n g t h o f t h e a g g r e g a t e i t s e l f . T h i s t r e n d e v i d e n c e s i t s e l f i n t h e newer d e s i g n s w h i c h c a l l f o r t h i n n e r d e n s e - m i x w e a r i n g s u r f a c e s and t h i c k e r c o a r s e - g r a d e d m i x b i n d e r c o u r s e s .

The p e r c e n t a s p h a l t r e q u i r e d i n a m i x t u r e may be d e t e r m i n e d by a s t a ­b i l i t y t e s t . The amount o f a s p h a l t r e q u i r e d t o s e c u r e t h e m o s t d e s i r ­a b l e v a l u e s f o r s o l i d vo lume d e n s i t y , u n i t w e i g h t d e n s i t y , p e r c e n t v o i d s i n t h e t o t a l m i x , and p e r c e n t v o i d s f i l l e d w i t h a s p h a l t s h o u l d be d e ­t e r m i n e d and c o n s i d e r e d i n m a k i n g t h e f i n a l o r o p t i m u m a s p h a l t s e l e c ­t i o n . P a s t e x p e r i e n c e has d e m o n ­s t r a t e d how n e c e s s a r y i t i s t h a t b i t u m i n o u s m i x t u r e s must he d e s i g n e d t o have a d e q u a t e w o r k a b i l i t y , d u r a ­b i l i t y and d e n s i t y .

40

The pu rpose o f t h i s i n v e s t i g a t i o n was t o c o r r e l a t e s u r f a c e r o u g h n e s s and s k i d r e s i s t a n c e o f t h e f i n i s h e d p a v e m e n t w i t h d e s i g n o f t h e b i t u ­minous pavement m i x t u r e s .

PREPARING LABORATORY TEST SPECIMENS FOR SAND-ASPHALT PAVEMENTS USING THE

HUBBARD-FIELD METOOD

The f o l l o w i n g method c a l l s f o r the p r e p a r a t i o n o f t e s t s p e c i m e n s i n b a t c h e s o f t w o b r i q u e t t e s , e a c h b a t c h w e i g h i n g 2 0 0 t o 230 g r a m s .

Weigh o u t t h e d r y sand and f i l l e r t o 0 . 1 g . Hea t t h e a g g r e g a t e i n t h e t a r e d m i x i n g d i s h t o 3 5 0 ° F . , and m i x t h o r o u g h l y . P l a c e t h e d i s h and c o n t e n t s on b a l a n c e and w e i g h i n t o i t t h e a s p h a l t h e a t e d t o 3 5 0 " F . T h o r o u g h l y m i x t h e c o n t e n t s o f t h e d i s h f o r 3 t o 5 m i n u t e s , o r as may be n e c e s s a r y t o i n s u r e t h o r o u g h i n ­c o r p o r a t i o n o f t h e a s p h a l t , u s i n g a 1 - i n . p u t t y k n i f e as a m i x e r and f i n i s h i n g t h e m i x i n g w i t h an i r o n i n g p r e s s u r e b e t w e e n t h e f l a t b l a d e o f t h e k n i f e and t h e s i d e o f d i s h , t o i r o n o u t a l l l u m p s . Then p l a c e a p ­p r o x i m a t e l y 100 G. o f m i x t u r e i n each o f two f o r m i n g m o l d s , numbered 1 and 2 , p r e h e a t e d t o 2 5 0 ° F . P l a c e t h e m o l d s w i t h base p l a t e s on a h o t p l a t e and b r i n g t h e t e m p e r a t u r e o f t h e c o n t e n t s o f N o . 1 m o l d t o 2 5 0 -2 6 0 ° F . as d e t e r m i n e d b y a t h e r ­mometer used f o r s t i r r i n g t he c h a r g e . Remove m o l d No. 1 f r o m h o t p l a t e and tamp t h o r o u g h l y w i t h t h e N o . 1 tamper ( b l a d e t y p e ) , u s i n g 60 o r more heavy b l o w s . F i n i s h t a m p i n g w i t h N o . 2 t a m p e r ( c y l i n d r i c a l t y p e , w e i g h i n g a p p r o x i m a t e l y 1400 grams) u s i n g 15-20 heavy b l o w s . Then i n s e r t t h e p l u n g e r and p l a c e t h e m o l d i n t he empty w a t e r b a t h on t h e base o f t h e c o m p r e s s i o n m a c h i n e . A p p l y p r e s s u r e t o t he t o p o f p l u n g e r u n t i l a l o a d o f 3000 p s i . o r a d i a l r e a d i n g o f 9 , 4 2 5 l b s . i s r e a c h e d . F i l l t h e b a t h w i t h c o l d w a t e r t o a d e p t h o f 3 m . and a l l o w b r i q u e t t e t o c o o l unde r p r e s s u r e f o r 5 m i n u t e s . Then r e l e a s e p r e s s u r e , r e m o v e t h e m o l d f r o m t h e b a t h and

f o r c e o u t b r i q u e t t e . W h i l e t h e f i r s t b r i q u e t t e i s u n d e r p r e s s u r e , b r i n g t he c h a r g e i n No . 2 m o l d t o 2 5 0 - 2 6 0 ° F . Tamp, c o m p r e s s , and c o o l as i n t h e ca se o f N o . 1 . The c o m p r e s s e d b r i q u e t t e s s h o u l d m e a s u r e a p p r o x ­i m a t e l y one i n c h i n h e i g h t . A f t e r b e i n g removed f r o m t h e m o l d , t h e t o p o f e a c h b r i q u e t t e i s m a r k e d f o r i d e n t i f i c a t i o n w i t h y e l l o w c r a y o n . The b r i q u e t t e s a r e a l l o w e d t o c u r e i n l a b o r a t o r y a i r o v e r n i g h t and a r e t h e n p l a c e d i n a w a t e r b a t h a c c u r ­a t e l y m a i n t a i n e d a t t h e t e m p e r a t u r e o f t e s t , u s u a l l y 1 4 0 ° F . , f o r one hou r b e f o r e t e s t i n g .

MAKING THE STABILITY TEST

Each spec imen i s t e s t e d by p l a c i n g I t , o r i g i n a l t o p end d o w n , m t h e t e s t i n g i r o l d a g a i n s t t he s h o u l d e r o f t he o r i f i c e and i n s e r t i n g t h e p l u n g ­e r . T h e c o m p r e s s i o n m a c h i n e i s o p e r a t e d so as t o p r o d u c e a u n i f o r m movement o f t h e t e s t i n g p l u n g e r a t t h e r a t e o f one i n c h i n 25 s e c o n d s . As t h e t e s t m i x t u r e i s l o a d e d , i t commences t o d i s t o r t a t t h e o r i f i c e i n t h e t e s t i n g r i n g , and t h e d i a l p o i n t e r r i s e s q u i t e r a p i d l y t o a m a x i m u m j u s t b e f o r e t h e b o n d i s b r o k e n . The maximum l o a d r e g i s t e r e d I S r e c o r d e d as t h e s t a b i l i t y v a l u e o f t h e s p e c i m e n . A f t e r t e s t i n g , t h e m i x t u r e i s r e m o v e d f r o m t h e m o l d w h i c h I S c l e a n e d b e f o r e i n s e r t i n g t h e n e x t s p e c i m e n . The t e s t appea r s t o be o f g e n e r a l l y s a t i s f a c t o r y r e ­p e a t a b i l i t y and b e t w e e n t e s t s p e c i ­mens o f t h e same b a t c h w i l l u s u a l l y show a v a r i a t i o n o f l e s s t h a n 10 p e r c e n t . The a v e r a g e o f t w o t e s t s i s r e p o r t e d as t h e s t a b i l i t y v a l u e o f t h e m i x t u r e . Ca re mus t be t a k e n t o m a i n t a i n u n i f o r m t e m p e r a t u r e o f b a t c h and b r i q u e t t e s d u r i n g t e s t .

A d e t e r m i n a t i o n o f t h e s p e c i f i c g r a v i t y o f a l l t e s t s p e c i m e n s i s made p r i o r t o s u b j e c t i n g them t o t h e s t a b i l i t y t e s t . T h i s s e r v e s as a check upon u n i f o r m i t y o f p r e p a r a t i o n and c o m p r e s s i o n , and t h e s p e c i f i c g r a v i t y r e s u l t s a re used i n c o m p u t i n g

4 1

t h e v o i d s i n t h e t e s t s p e c i m e n and v o i d s i n m i n e r a l a g g r e g a t e .

S p e c i f i c g r a v i t y i s d e t e r m i n e d by w e i g h i n g t he b r i q u e t t e , f i r s t i n a i r and t h e n submerged i n w a t e r a t 7 7 ° F . , a l l w e i g h t s b e i n g r e c o r d e d t o t h e n e a r e s t 0 . 1 p r a m . I f A = w e i g h t i n a i r and B = w e i g h t i n w a t e r , s p e c i f i c g r a v i t y i s c a l c u l a t e d a c c o r d i n g t o t he f o l l o w i n g f o r m u l a -

Sp. C r . = ^

VOIDS I N COMPRESSED PAVING MIXTURE

The p e r c e n t o f v o i d s i n t h e com­p r e s s e d p a v i n g m i x t u r e may be r e a d i l y a s c e r t a i n e d f r o m i t s s p e c i f i c g r a v i t y i f t h e s p e c i f i c g r a v i t y and p e r c e n t b y w e i g h t o f e a c h c o n s t i t u e n t a r e k n o w n . I n c o n d u c t i n g any s e r i e s o f s t a b i l i t y t e s t s on s a n d - a s p h a l t a g g r e g a t e s , t h e s p e c i f i c g r a v i t y o f t h e s a n d , m i n e r a l f i l l e r , and a s ­p h a l t c e m e n t s h o u l d t h e r e f o r e be d e t e r m i n e d b y t h e m e t h o d s o f t h e A m e r i c a n S o c i e t y f o r T e s t i n g M a t e r i ­a l s f o r a p p a r e n t s p e c i f i c g r a v i t y .

I f W, W j , and W2 r e p r e s e n t t h e p e r c e n t a g e w e i g h t s and G, G^ ,̂ and G2 r e p r e s e n t t h e s p e c i f i c g r a v i t i e s o f t he s and , m i n e r a l f i l l e r , and a s p h a l t r e s p e c t i v e l y o f a g i v e n m i x t u r e , t h e t h e o r e t i c a l maximum d e n s i t y o f such a m i x t u r e , i f i t w e r e f r e e f r o m v o i d s , i f f i r s t d e t e r m i n e d by t h e f o J l o w i n p f o r m u l a where D r e p r e s e n t s t h e t h e o r e t i c a l max imum d e n s i t y .

D - 100

G Gi

VOIDS I N MINERAL AGGREGATE OF COMPRESSED MIXTURE

P e r c e n t o f v o i d s i n t h e m i n e r a l a g g r e g a t e , d e s i g n a t e d V . M . A . , o f a c o m p r e s s e d a s p h a l t p a v i n g m i x t u r e , e x c l u d i n g t h e a s p h a l t i t s e l f , i s r e a d i l y c a l c u l a t e d f r o m t h e d a t a u s e d m c a l c u l a t i n g v o i d s i n t h e m i x t u r e . T h u s , i f a c o m p r e s s e d m i x t u r e has a s p e c i f i c g r a v i t y o f 2 . 2 0 a n d c o n t a i n s 10 p e r c e n t o f a s p h a l t b y w e i g h t , i t i s e v i d e n t t h a t t h e a p p a r e n t s p e c i f i c g r a v i t y o f t h e m i n e r a l a g g r e g a t e e q u a l s 2 . 2 0 - . 2 2 o r 1 . 9 8 . I f g = t h e a p p a r e n t s p e c i f i c g r a v i t y o f t h e m i n e r a l a g g r e g a t e , t h e f o r m u l a w o u l d be

The a c t u a l s p e c i f i c g r a v i t y o f a t e s t spec imen h a v i n g been d e t e r m i n e d i t s p e r c e n t a g e o f v o i d s i s now r e a d i l y c a l c u l a t e d by t h e f o l l o w i n g f o r m u l a i n w h i c h D = t h e o r e t i c a l max imum d e n s i t y , d - a c t u a l s p e c i f i c g r a v i t y and V = p e r c e n t o f v o i d s .

u . 1 0 0 ^ ( D - ^

g = d - ( d X ) o r d ( 1 -

100

l2_ 100

)

The maximum t h e o r e t i c a l d e n s i t y o f t h e m i n e r a l a g g r e g a t e ( s a n d and m i n e r a l f i l l e r ) r e p r e s e n t e d by D w o u l d be c a l c u l a t e d by t h e f o r m u l a

W • Wj

W w7 77- + — i

A n d t h e V . M . A . , o r p e r c e n t o f v o l d s , i n t h e c o m p r e s s e d m i n e r a l a g g r e g a t e w o u l d t h e n be

V . M . A . 100 ( D , • g )

D

I n d e s i g n i n g an a s p h a l t p a v i n g m i x t u r e , t h e V . M . A . s h o u l d a l w a y s be c o n s i d e r e d , as v a r i a t i o n s i n p e r ­c e n t a g e o f a s p h a l t may s e r i o u s l y i n t e r f e r e w i t h o b t a i n i n g t h e c l o s e p a c k i n g o f t h e a g g r e g a t e w h i c h i t I S d e s i r a b l e t o s e c u r e .

S T A B I L I T Y TESTS PERFCWMED USING THE M O D I F I E D HUBBARD-FIELD METHOD FOR

ASPHALTIC CONCRETE

T e s t s p e r f o r m e d u s i n g t h e H u b b a r d -F i e l d method were m a c c o r d a n c e w i t h t e s t p r o c e d u r e o u t l i n e d i n a pam­p h l e t e n t i t l e d "The R a t i o n a l D e s i g n

42

o f A s p h a l t P a v i n g M i x t u r e s " p u b ­l i s h e d by t h e A s p h a l t I n s t i t u t e on O c t o b e r 13 , 1935 .

The H u b b a r d - F i e l d s t a b i l i t y t e s t d e v e l o p e d i n t h e l a b o r a t o r y o f t h e A s p h a l t I n s t i t u t e was o r i g i n a l l y d e s i g n e d t o d e t e r m i n e t h e r e s i s t a n c e t o d e f o r m a t i o n o f s h e e t a s p h a l t p a v ­i n g m i x t u r e s . I t s u s e , w i t h o u t m o d i f i c a t i o n , has been e x t e n d e d t o s a t i s f a c t o r i l y c o v e r s t o n e f i l l e d s h e e t a s p h a l t o r f i n e g r a d e d a g ­g r e g a t e a s p h a l t i c c o n c r e t e i n w h i c h t h e maximum d i a m e t e r o f a g g r e g a t e p a r t i c l e d o e s n o t e x c e e d 5 / 8 i n . , and t h e p e r c e n t a g e o f a g g r e g a t e r e ­t a i n e d on t h e 10-mesh s i e v e does n o t e x c e e d 35 p e r c e n t o f t h e t o t a l . The 2 - i n . d i a m e t e r t e s t spec imen and t e s t i n g m o l d , h o w e v e r , make i t i m ­p o s s i b l e t o t e s t o t h e r m i x t u r e s i n common use w h e r e b o t h t h e maximum s i z e and p e r c e n t a g e o f a g g r e g a t e r e ­t a i n e d on t h e 10-mesh s i e v e exceeds t h e s e l i m i t s . Thus c o a r s e a g g r e g a t e a s p h a l t c o n c r e t e and m i x t u r e s o f s t o n e o r g r a v e l p r o d u c t s w i t h r o a d o i l s a n d c u t - b a c k s , s u c h as a r e w i d e l y u s e d i n l o w - c o s t r o a d c o n ­s t r u c t i o n , c a n n o t be e v a l u a t e d b y t h i s t e s t .

A s p e c i a l m o l d and t e s t i n g r i n g f o r t h e c o a r s e r m i x t u r e s have been d e v e l o p e d w h i c h p r o m i s e t o be a u s e ­f u l means o f d e t e r m i n i n g t h e i r r e l ­a t i v e s t a b i l i t y i n v a l u e s w h i c h a r e d i r e c t l y c o m p a r a b l e t o t h o s e o b ­t a i n e d on s h e e t - a s p h a l t m i x t u r e s m t h e s m a l l e r m o l d . T h i s e q u i p m e n t may be c o n s i d e r e d as an a d j u n c t t o t h e o r i g i n a l s t a b i l i t y t e s t . W h i l e , w i t h c e r t a i n m o d i f i c a t i o n s i n method o f p r e p a r i n g t e s t s p e c i m e n s , s h e e t -a s p h a l t m i x t u r e s w i l l show t h e same r a n g e o f s t a b i l i t y when t e s t e d i n t h e l a r g e m o l d as i n t h e s t a n d a r d 2 - i n . m o l d . The l a r g e r s i z e i s n o t c o n v e n i e n t f o r s h e e t - a s p h a l t and i s i n t e n d e d o n l y f o r use w i t h t he c o a r s e a g g r e g a t e m i x t u r e s . T h i s e q u i p m e n t c o n s i s t s o f t h e f o l l o w i n g p i e c e s :

( 1 ) A c y l i n d r i c a l s t e e l f o r m i n g and t e s t i n g m o l d o f a p p r o x i m a t e l y 1/2 i n . w a l l t h i c k n e s s , a c c u r a t e l y

machined t o 6 i n c h i n t e r n a l d i a m e t e r a n d m e a s u r i n g 6 i n . i n h e i g h t .

( 2 ) A t e s t i n g r i n g o f t e m p e r e d s t e e l 3 / 8 i n . t h i c k and o f t h e same o u t s i d e d i a m e t e r as t h e m o l d o r a p p r o x i m a t e l y 7 i n . T h e m i n i m u m o r i f i c e o f t h e t e s t i n g r i n g , w h i c h i s 5 - 3 / 4 m . e x t e n d s f o r a d e p t h o f 1/8 i n . a f t e r w h i c h i t t a p e r s o u t t o a d i a m e t e r o f 6 i n . a t t h e b o t t o m .

( 3 ) A r i n g c l a m p w i t h s e t s c r e w s t o h o l d t h e t e s t i n g r i n g t i g h t l y a g a i n s t t h e b o t t o m o f t h e t e s t i n g m o l d .

( 4 ) A p l u n g e r c o n s i s t i n g o f a c i r c u l a r s t e e l p l a t e 5 - 3 / 4 i n . i n d i a m e t e r i n t h e c e n t e r o f w h i c h i s s c r e w e d a 1 - 1 / 4 i n . d i a m e t e r r o u n d s t e e l r o d 4 - 3 / 4 i n . l o n g . The t o p o f t h i s r o d c o n t a i n s a d e p r e s s i o n i n t o w h i c h the b a l l end o f t h e t e s t ­i n g h e a d o f t h e s t a b i l i t y m a c h i n e f i t s . I t a l s o c a r r i e s a p r o j e c t i n g p m w h i c h f i t s i n t o a s l o t i n a m e t a l t i p p e d h a n d l e w h i c h may be a t t a c h e d when I t i s d e s i r e d t o use t h e p l u n g e r as a t a m p e r .

( 5 ) A m e t a l b a s e p l a t e 12 i n . square and 1 i n . t h i c k .

( 6 ) A p o r t a b l e , m e t a l w a t e r b a t h 1 4 - 1 / 2 i n . l o n g , 12 i n . w i d e , and 9 i n . d e e p t o t h e b o t t o m o f w h i c h a re f a s t e n e d a number o f s t o u t m e t a l r e i n f o r c i n g r i b s .

( 7 ) A t ampe r d e s i g n a t e d as No . 2 c o n s i s t i n g o f 1 . 7 5 i n . d i a m e t e r s t e e l r o d 3 i n . i n l e n g t h w i t h a h a r d e n e d s t e e l p l a t e 1 . 8 7 5 i n . i n d i a m e t e r a t t a c h e d t o t h e s t e e l r o d w i t h c o u n t e r s u n k s c r e w s . A 0 . 5 i n . s t e e l r o d e n c a s e d i n a 1.5 i n . wood h a n d l e 7 . 3 i n . l o n g i s t h e a t t a c h e d h a n d l e . A p p r o x i m a t e w e i g h t liOO grams.

( 8 ) A t amper d e s i g n a t e d as No . 3 i s a p l u n g e r c o n s i s t i n g o f a c i r ­c u l a r s t e e l p l a t e 5 . 7 5 i n . i n d i a m ­e t e r , i n t h e c e n t e r o f w h i c h i s s c r e w e d a 1 - 1 / 4 i n . d i a m e t e r r o u n d s t e e l r o d 4 - 3 / 4 i n . l o n g . The t o p o f t h i s r o d c o n t a i n s a d e p r e s s i o n i n t o w h i c h t he b a l l end o f t h e t e s t ­i n g head o f t h e c o m p r e s s i o n mach ine f i t s . I t c a r r i e s a p r o j e c t i n g p i n

43

w h i c h f i t s i n t o a s l o t i n a m e t a l -t i p p e d h a n d l e w h i c h i s a t t a c h e d when i t I S d e s i r e d t o use t h e p l u n g e r as a t amper .

PROCEDURE

A l l m i x t u r e s a r e b l e n d e d i n a 3 - q u a r t a l u m i n u m s t e w p a n w i t h a 1 - i n . b l a d e p u t t y k n i f e . The N o . 2 tamper f r o m the r e g u l a r e q u i p m e n t i s used i n a d d i t i o n t o t h e new t a m p e r w h i c h I S d e s i g n a t e d as N o . 3 , f o r p l a c i n g t h e m i x t u r e s i n t h e m o l d .

Mixing - I n t o t h e t a r e d d i s h i s w e i g h e d t h e s e v e r a l m i n e r a l c o n ­s t i t u e n t s o f t h e m i x so as t o p r o ­duce a t o t a l w e i g h t o f 2 , 0 0 0 grams o f a g g r e g a t e . A f t e r t h o r o u g h d r y m i x i n g , t h e d i s h and c o n t e n t s a r e p l a c e d on a h o t p l a t e and b r o u g h t t o t h e d e s i r e d t e m p e r a t u r e , 300 t o 3 5 0 ° F . T h e a g g r e g a t e i s a g a i n s t i r r e d , r e - w e i g h e d , and a d j u s t e d f o r any change i n w e i g h t . W i t h t h e p u t t y k n i f e , a c r a t e r i s f o r m e d i n t h e a g g r e g a t e t o h o l d t he h o t a s p h a l t w h i c h i s h e a t e d t o 3 0 0 ° F . and c a r e ­f u l l y we ighed i n by p o u r i n g f r o m the can i n w h i c h i t was h e a t e d . M i x i n g i s a c c o m p l i s h e d by s t i r r i n g the mass, b e g i n n i n g f i r s t w i t h t he p u t t y k n i f e a t t h e c e n t e r and s t i r r i n g t h e c e n ­t r a l p o r t i o n i n s u c h manne r as t o p r e v e n t c o n t a c t o f a s p h a l t w i t h t h e d i s h u n t i l t h e a g g r e g a t e has a b s o r b e d t h e main p o r t i o n . The m o t i o n o f t h e b l a d e may now be changed t o p r o d u c e an i r o n i n g e f f e c t t h a t w i l l b l a d e o u t any l umps f o r m e d and a s s i s t i n t h o r o u g h d i s t r i b u t i o n o f t he a s p h a l t . The m i x i n g i s f i n i s h e d by a f i n a l s t i r r i n g o f t h e e n t i r e mass f r o m the d i s h w a l l i n w a r d . The w h o l e m i x i n g o p e r a t i o n i s f i n i s h e d i n f r o m 5 t o 6 m i n u t e s .

C o n p r e s s i o n - The m i x i s i m m e d i a t e l y t r a n s f e r r e d t o t h e m o l d w h i c h , w i t h t h e base p l a t e , has been p r e v i o u s l y warmed t o a t e m p e r a t u r e o f 100 t o 1 2 5 ° F . A b o u t h a l f t h e m i x i s p l a c e d i n t h e m o l d u s i n g a h o t s p o o n . The

p u t t y k n i f e i s a g a i n u s e d a n d t h e mass r e s t i r r e d i n t h e m o l d w i t h c a r e ­f u l b l a d i n g a t t h e s i d e s t o p r e v e n t t h e f o r m a t i o n o f p o c k e t s . T h i s p o r ­t i o n i s tamped w i t h t h e No . 2 t amper , 50-60 b l o w s . The b a l a n c e o f t he m i x i s t h e n p l a c e d m t h e m o l d , s t i r r e d , and b l a d e d as b e f o r e , t amped 5 0 - 6 0 b l o w s w i t h t h e N o . 2 t a m p e r , and 50 b l o w s o f t h e N o . 3 t a m p e r . T h e t e m p e r a t u r e r e c o r d e d by a t he rmomete r f o r c e d i n t o t h e m i d d l e o f t h e b r i ­q u e t t e s h o u l d r e a d a b o u t 1 0 0 ° C. The b r i q u e t t e i s a l l o w e d t o c o o l t o 8 0 ° C. when a f i n a l t a m p i n g o f 50 b l o w s o f t h e No . 3 t a m p e r i s g i v e n . The b r i q u e t t e i s now c o o l e d i n t h e m o l d by i m m e r s i o n i n a c o l d w a t e r b a t h a t a p p r o x i m a t e l y 7 2 ° F . ( o r r o o m t e m p e r a t u r e ) . The b r i q u e t t e may t h e n be f o r c e d o u t e a s i l y by hand w i t h t he N o . 3 t a m p e r .

S p e c i f i c Gravity - S p e c i f i c g r a v i t y o f t h e c o m p r e s s e d m i x t u r e may be c a l c u l a t e d by d i s p l a c e m e n t o f w a t e r , o r by v o l u m e d e t e r m i n a t i o n , u s i n g an Ames d i a l t o measu re t h e h e i g h t o f t h e s p e c i m e n as m o l d e d . V o i d s i n t h e m i n e r a l a g g r e g a t e and v o i d s i n t h e m i x a r e c a l c u l a t e d b y t h e method d e s c r i b e d i n c o n n e c t i o n w i t h t he o r i g i n a l s t a b i l i t y t e s t . S t a b i l ­i t y i s d e t e r m i n e d a t 1 4 0 ° F . by p l a c ­i n g t h e m o l d and b r i q u e t t e i n t h e w a t e r b a t h a t 1 4 0 ° F . f o r one h o u r , t h e n b r i n g i n g t h e a s s e m b l y i n p l a c e on t h e t e s t i n g r i n g u n d e r t h e head o f the c o m p r e s s i o n machine and a p p l y ­i n g t h e l o a d a t t h e s t a n d a r d r a t e o f 1 i n . i n 25 s e c o n d s and n o t i n g t h e maximum l o a d r e q u i r e d t o cause f a i l u r e .

SKID RESISTANCE MEASUREMENTS

The s k i d r e s i s t a n c e measurements w e r e made by t h e f o r w a r d s k i d d i n g , s t o p p i n g d i s t a n c e m e t h o d . T h i s p r o ­cedure has p r e v i o u s l y been d e s c r i b e d by P r o f e s s o r Moyer ( 1 ) ^ and S h e l b u r n e and Sheppe ( 2 ) . ^ Numbers i n p a r e n t h e s e s r e f e r t o r e f e r e n c e s a t the end o f t he p a p e r .

44

A 1948 Flee tmas ter Chev ro l e t car weighing 3100 l b s . and equipped w i t h 6.00 by 16 i n . 4 - p l y t i r e s was used throughout the teat*. P r i o r to per­f o r m i n g the t e s t a the speedometer waa c a l i b r a t e d by means o f a s t o p ­watch over two measured m i l e s . S i x c a l i b r a t i o n runs were made a t each speedometer r e a d i n g o f 20, 30, and 40 mph and the t ime checked c a r e ­f u l l y . R e s u l t s o f the c a l i b r a t i o n are shown g r a p h i c a l l y i n F igu re 1 .

A l l s k i d measurements were per ­formed on tangen ts w i t h r e l a t i v e l y even su r f aces and on l e v e l s ec t i ons o f pavement. Befo re each s e r i e s o f t e s t s , the a i r pressure i n each t i r e was a d j u s t e d t o 30 p . s . i . I n per ­fo rming the tests the car was brought t o an i n i t i a l u n i f o r m speed o f 20, 30, or 40 mph and the brakes app l i ed h a r d i n s t a n t l y t o l o c k a l l f o u r whee l s . At the i n s t a n t the wheels were l o c k e d , an e l e c t r i c d e t o n a t o r l o a d e d w i t h a 22 b l a n k c a r t r i d g e f i r e d a c h a l k b u l l e t on the pave­ment. The gun was a t t a c h e d t o the f r o n t bumper o f the car and actuated e l e c t r i c a l l y through the brake pedal . The d i s t a n c e f rom the cha lk mark to the gun was measured by means o f a s t ee l tape. Three measurements were made a t each speed and r e s u l t s r e ­por ted are average va lues .

1

A

/» 'S »o »»• 30 31 «»

i^eoomfrrtf Kaunie (MM)

F i g u r e 1 Speedome te r C a l i b r a t i o n

Four se t s o f new n a t u r a l rubber t i r e s were used i n the t e s t s . They were replaced at the end o f 5000 m i . At t h i a mileage the t read was s t i l l very good.

In pe r fo rming the t e s t s , measure­ments were made a t the th ree speeds w i t h the sur face i n a dry c o n d i t i o n . The pavement was then f l o o d e d w i t h water and the t e s t s repea ted . Th i s method o f p r o d u c i n g a wet pavement i s b e l i e v e d t o be c o m p a r a b l e t o n a t u r a l wet weather c o n d i t i o n s . A f t e r each o f the three measurements, wet, a t one speed, the f l o o d i n g o f the su r f ace w i t h water was repea ted . By t h i s method i t was poss ib le t o main­t a i n u n i f o r j n l y wet su r f aces .

As a m a t t e r o f r e c o r d bo th a i r and pavement temperatures were taken f o r each t e s t . Since the t e s t s were p e r f o r m e d i n t e r m i t t e n t l y d u r i n g a 10-month p e r i o d , a i r t e m p e r a t u r e s ranged f r o m 3 4 ° t o 9 6 ° F . and av­eraged 78° F. Corresponding pavement t e m p e r a t u r e s v a r i e d f r o m 3 7 ° t o 110° F. and averaged 83° F.

The same d r i v e r was used through­o u t the s e r i e s o f t e s t s . The car was maintained i n excel l e n t ope ra t ing c o n d i t i o n . T h i s r e q u i r e d f r e q u e n t b a l a n c i n g o f wheels and a d j u s t i n g o f b r a k e s . The roundness o f t he t i r e s was a l so checked. A t the 40 mph on a wet s u r f a c e , there was some s i d e s k i d d i n g as the c a r came t o r e s t . Check t e s t s were made u n t i l the s i d e s k i d d i n g was e l i m i n a t e d .

When these t e s t s were conducted on the p r i m a r y highway system, the State Highway P a t r o l d i r e c t e d t r a f f i c d u r i n g the t e s t s .

DESCRIPTION OF SURFACES TESTED FOR SKID RESISTANCE

The sur faces tes ted were se lec ted w i t h the v iew o f d e t e r m i n i n g s k i d r e s i s t a n c e q u a l i t i e s o f t h e two t y p e s o f pavement , n a m e l y , sand-a s p h a l t and b i t u m i n o u s c o n c r e t e . I t I S r e a l i z e d t h a t t h e r e i s c o n ­s i d e r a b l e v a r i a t i o n i n s u r f a c e s o f the same type , due p r i m a r i l y t o as-

45

p h a l t c o n t e n t and t h e a g g r e g a t e c h a r a c t e r i s t i c s such as p a r t i c l e shape and g r a d a t i o n .

The pavements t e s t e d were two lanes i n w i d t h . A l l t e s t s were made i n the d i r e c t i o n o f t r a f f i c and i n the t r a f f i c l a n e s , b u t n o t i n the c e n t e r o f the h i g h w a y . A l l t e s t s on each s u r f a c e were made i n the same lane and d i r e c t i o n , so as t o take advantage o f al ignment and grade.

On the b a s i s o f average 2 4 - h r . counts , t r a f f i c on the roads t e s t ed v a r i e d f r o m 100 v e h i c l e s t o 6000

per day. For the year end ing December 3 1 ,

1949, an average o f the t r a f f i c counts a t the 43 l o c a t i o n s was 1870 v e h i c l e s . S i n c e 42 p e r c e n t o f t h e s e s a n d -a s p h a l t s u r f a c e s are on secondary r o a d s , t he t r a f f i c c o u n t i s c o n ­s i d e r a b l y less than f o r the p r o j e c t s on the pr imary system.

The s a n d - a s p h a l t pavement c o n ­s i s t s o f one o r more c o u r s e s o f sand-asphalt l a i d on a prepared s o i l type base course which i s cons t ruc ted according to standard s p e c i f i c a t i o n s .

TYPICAL GRADATION

SAND ASPHALT

N. C. Pro jec t

3-6-30-132 3-7-28-15 3-7-28-16 3-6-31-123 1- 7-6-85 2- 102

2-115

N C. Spec i f i ca t i on Special Section No. Provisions

Percent Passing Larger Sieve and

125-1.2 125-1.2 125-1.2 125-1.2 125-1.2 125-1.2

125 No. 10 40 80

200 -200 B i t .

5% Min.-200 5% Min.-200 5% Min -200 5% Min.-200 5% Min.-200 5% Min.-200

% Retained 0-15 0-60

20-80 0-50 0-12

6X-7K

Grading B 11 B B B B

Numbers 40 80 200 -200

38.90 26.70 15.20 7.10 21.40 55 20 10.10 5.53 21.50 54.10 10.30 6.04 17.50 46.70 21.90 5.90

7.83 0.70 7.50 26.20 29.00 22.10 6.67 8 00 0.20 6.20 43.90 34.60 7.10

Pet. AP-3 7.77 7 77 7.76 8 00

Retained on 10

1.60

0 30

7.01 0.60 6.70 44.50 40.10 1.09

BITUMINOUS CXWCBETE

1706 4771 4772 4722 3530

10-6-96-141

140 126 126 126 126

125

(2-28-50) 25% Min -No. 10

5% Min.-200 5% Min.-200 AP-5 Used instead of AP-3 % Retained 3/4-4 0-10 4-10 0-30

20-60 10-60

5-30 2-8

6%-6H

5.72 27.35 10 50 A 6 00 17.00 18.35 A 5.70 17.40 14.27 A 5 73 34 15 16.95 A (AP-5) 31.95 20.40

6.03

6.71 25.00 16. 10

10-40 40-80 80-200

-200 AP-3

54.20 57 55 54 10 37.05 35.15

2 23 1.10 8.53 6 12 6.47

50 00 2 19

46

on o l d b i t u m i n o u s s u r f a c e t r e a t e d roads , on o l d cement concre te pave­ments, or o l d and new t r a f f i c bound s t o n e bases . I t i s a h o t p l a n t -mixed m a t e r i a l composed o f 85-100 p e n e t r a t i o n a s p h a l t i c cement i n the amount o f 5.0 t o 9.0 p e r c e n t com­b ined w i t h n a t u r a l non-commerc ia l or commercial sand, or a combination o f sand and s tone s c r e e n i n g s . A l l aggregates pass a No. 4-mesh s i eve w i t h 0 t o 10 p e r c e n t p a s s i n g the No. 200-mesh s i eve . The Nor th Caro­l i n a s p e c i f i c a t i o n s do n o t r e q u i r e a s t a b i l i t y t e s t . N a t u r a l n o n ­commercial sand i s the most commonly used a g g r e g a t e f o r t h i s t y p e o f su r f ace .

The a s p h a l t i c c o n c r e t e pavement c o n s i s t s o f a f a i r l y dense-graded b i t u m i n o u s s u r f a c e . These wea r ing courses are u s u a l l y about one i n c h t h i c k and p l a c e d on a b i n d e r o r l e v e l i n g course u s u a l l y f rom one t o two inches i n t h i cknes s . The binder i s a ho t p l a n t - m i x e d m a t e r i a l com­posed o f 85-100 p e n e t r a t i o n a s p h a l t i c cement (5 .0 to 8.0 percen t ) combined w i t h crushed stone or g rave l coarse aggregate and sand or stone screen­i n g s , or a combina t ion t h e r e o f . I n some cases , a non -commerc i a l sand i s combined w i t h the coarse aggre­g a t e . The aggrega te i s u n i f o r m l y

I

I I Ut^tai/h, HUH

('•*> (0-3) Pmnmmt T f f t <m/ / I f t n fe^j

F i g u r e 2 Roughness V a l u e s f o r t he t w o t y p e s o f B i t u m i n o u s P a v e n i e n t

s u r f a c e s i n N o r t h C a r o l i n a

graded f rom coarse t o f i n e w i t h 100 pe rcen t pass ing the 3 / 4 - i n c h s ieve and 5 percent t o 10 percent pass ing the No. 200-mesh s i e v e . S p e c i f i c a ­t i o n s and S p e c i a l P r o v i s i o n s f o r t h i s type s u r f a c e are v a r i e d w i t h i n the above l i m i t s i n order to o b t a i n t h e most d e s i r a b l e m i x w i t h t h e ma te r i a l s a v a i l a b l e .

The b i tuminous concre te su r faces s t u d i e d were f rom one to f i v e years o l d .

DISCUSSION OF SKID RESISTANCE TEST DATA

The complete data f o r more than 1500 s t o p p i n g d i s t a n c e t e s t s on 43 pavement s u r f a c e s are presented i n Table 1 . These s t o p p i n g d i s t a n c e s are a c t u a l average f o r w a r d s k i d d i n g d i s t ances and do no t i n c l u d e d r i v e r r e a c t i o n t ime . Table 1 inc ludes the average c o e f f i c i e n t o f f r i c t i o n as computed from the f o l l o w i n g standard s topping d is tance fo rmu la :

F = SOS

Where F = Average c o e f f i c i e n t o f f r i c t i o n ,

V = I n i t i a l speed i n m i l e s per hour a t the t ime o f a p p l y i n g brakes.

S = Average s t o p p i n g distance i n f e e t .

R e s u l t s a re g rouped a c c o r d i n g t o t h e two pavemen t t y p e s p r e ­v i o u s l y d e s c r i b e d f o r both d r y and wet s u r f a c e s . Average v a l u e s are g iven by groups o f each type shown i n Table 2 as w e l l as grand average f o r both types .

C o n s i d e r i n g data i n Table 1 f o r a speed o f 40 mph, s topping d is tances from 60.1 to 88.3 and averaging 68.7 f t . , were measured on dry s u r f a c e s . Corresponding computed c o e f f i c i e n t s o f f r i c t i o n ranged from 0.89 to 0.60 and a v e r a g e d 0 . 7 8 . Thus i t can r e a d i l y be seen t h a t a l l 43 sur faces have e x c e l l e n t s k i d r e s i s t a n c e when t e s t e d d r y .

Values f o r the same surfaces when

TABLE 1

SKID RESISTANCE

47

Projec t Route DRY

Speed Speed

MPII

2- 9-22-204 3- 6-28-126 3-9-31-204 3-7-28-15 3-7-28-16 2-102 2- 9-23-201 3- 9-32-203 3-6-31-123 6-9-57-210 3-6-28-122 3695 3334 3-6-30-132 1-7-6-85 2200 1- 9-8-201 2- 9-18-202 2-652 2-9-24-206 1076 2223

2- 9-24-209 1-9-12-201 6-9-57-209 3- 9-34-210

2-454 2 - l l S

2620 Craven Co.

Carteret Co. Pasquotank Co

Rowan Co.

Vance Co Cleveland Co.

Co Rd LS 17 Co Rd Co Rd Co Rd Co.Rd. NC 55 Co Rd. US 117 Co Rd US 17 NC 24 NC 211 NC 210 Co Rd US 117 Co Rd Co Rd. Co Rd Co Rd NC 97 NC 41 Co.Rd. Co Rd. Co Rd. Co Rd Co Rd Co Rd US 258 US 70 US 70 US 17 US 70

& 29 US 1 US 29

20 S D

17.9 15.5 18 3 16 16. 16 16 19 16.4 16 9 16 17 IS 25 16 15 15 15 IS 8 17 16 16 16. 16 16 18. 16. 18 23 24 23.5 23 3

16 4 25 6 16 9

C

75 86 73 83

.82 81 81 68 81 79 81 76 87 53 81 84

.86 85 84 76

.83 83 82 83 79 72 81

.71 56 55 57

.57

.81 52 79

30 S D 40 1 35 9 39.7 39.2 37 4 34 8 39 43, 36 57 38 38 36 50. 37 38 37 35 8 35 38 37. 37, 36, 37, 38 42 37 39 50 52 50 48.5

37 4 52 1 37.5

SAND-ASPHALT

Speed

40 S D. C

73.6 63 8 67.0 65 3 66.3 64 5 65 8 74 3 64 0 65 8 67 2 64.4 64 3 83 8 69.6 68 65 62 60 65 62 64 8 64.8 65 8 66.7 70 7 68 8 64 9 79 8 80.0 84 2 80 5

65 3 88 3 65.6

Speed WET

Speed Speed

20

75 84 76 77 80 86 76 69 83 80

.78 79

.83

.60

.80

.79 81 84

.85 78 79 81

.83

.81 78 71 80

.77

.60

.57 59

.62

80 57

.80

S.D.

72 21 84 20 80 19 82 17,

.81 17

.83 18,

.81 20,

.72 18

.83 33.

.81 21. 79 20. 83 22 83 17. 64 30

.77 17, 77 23

.82 16, 85 20

.89 20 81 16 85 18 82 17

.82 19. 81 17 80 20 76 20

.77 20

.82 19.

.67 25. 67 25 63 24.

.66 28

.82 60

.81

20 8 29.9 26 2

.62 66 68 75

.75 71

.66 73 65 63 65

.61

.76 43

.77

.56

.81 65

.64 81 72

.76 68 77 65

.65

.65 68 52

.52

.54 47

.64 45 51

30 S D

49 5 46. 45 42 37 38 46 47 66.0 46.4 49 2 49.5 42.8 70.0 40 2 53 9 48.7 52. 1 49 8 36 8 42 8 43.3 49 40 50 46 48 40 8 62 8 S3 9 57 4 55-7

49.8 65.8 64 2

C 61 65 67 71

.80

.78

.65 63

.45 65 61

.61 70 43 75 56 62

.58 60 82

.70

.69

.61 74

.61 65

.62 74

.48 56 52 54

.60 46 47

40 s .n

88 1 90 9 82.3 81.2 74.7 71.4 86 5 85.0 126.3 86.9 94.7 90.8 89.4 121.3 78.7

60 .59 .65 .66 71

.75

.62

.63 42 61

.56 59

.60 44 68

112.4 .47 82 9 89.5 87.2 68 7 79.7 80.4 85 4 73.9 94.4 81.4 86 8 76 4 108 S 99 3 99.3 107 4

95.6 111 0 127.7

.64 60

.61

.78 67 66

.62 72 56 65 61 70 49 54

.54

.SO

56 48 42

TOTAL 17 99 .758 40 11 759 68.99 .780 21 40 650 49 39 625 91 32 598

48

TABLE 1 C o n t i n u e d

ASPHALTIC OasaOETE 1706 US 301 15.3 .87 37 5 80 65.5 .81 23 0 58 59.0 .51 106.6 . 50 3530 C i t y Street 18 0 74 39.7 .76 67.7 .79 25.4 52 57.5 .52 111.4 . 48 4771 US 1 16 5 81 38 7 .76 66 6 .80 22.3 60 55.2 54 98 5 . 54 4772 US 1 16.8 .79 39.8 .75 68.2 .78 22 4 60 50.7 .59 90.3 . 59 4722 US 158 16.4 .81 39.4 .76 68.9 .77 17.0 .78 41.0 .73 81 1 . 66

10-6-96-141 US25 &70 19.0 70 40 5 .74 73 2 73 25 2 .53 63 1 .48 110.0 48 G u i l f o r d Co. US29 &70 16.9 .79 37.6 .80 64 6 .83 22 7 .59 55 6 .54 110 0 48 G u i l f o r d Co. US 70 18.2 73 38 3 .78 64 5 .83 20.3 .66 48 9 61 92 3 58

TOTAL 17.14 .780 38.94 769 67 40 .793 22 29 608 53 88 .565 100 03 .539

GRAND AVERAGE 17.83 .762 39 89 761 68 70 .782 21 57 .642 50 23 614 92 94 .587

TABLE 2

SKID RESISTANCE TCSTS AT DIFFERENT TCMPEHA'njHES

Pro jec t DRY WET

Speed Speed Speed Speed Speed Speed

MPH 20 30 40 20 30 40 Temp. S.D. C S.D. C S D C S D C S D C S.D C

4211 68° 17.4 .76 38.8 77 69 7 76 22.2 60 51 9 58 105.2 51 4211 84° 21.5 .62 54 2 .55 90.5 .58 32 0 42 72.5 .41 125.3 42

3-9-32-203 70° 17.8 .75 38.4 .78 66 3 81 18 0 74 43.0 .70 78 3 .69 3-9-32-203 98° 19.5 68 43 5 .69 74.3 72 18 2 .73 47 4 .63 85 0 63

S D. C

Skid Distance C o e f f i c i e n t

wet ( a t 40 mph ranged f r o m 71 .4 t o 127 .7 and ave raged 9 2 . 9 f t . The co r re spond ing computed c o e f f i c i e n t s o f f r i c t i o n v a r i e d from 0.75 to 0.42 and averaged 0.587. I f the surfaces are r a t e d acco rd ing t o p rev ious i n ­v e s t i g a t o r s on the basis o f s topp ing d is tances o f 133 f t . ( c o e f f i c i e n t o f f r i c t i o n 0 . 4 ) then a l l 43 s u r f a c e s would f a l l w i t h i n t h i s l i m i t . Ac­c o r d i n g t o AASHO d e s i g n s t a n d a r d s ( 5 ) i n which a f a c t o r o f s a f e t y o f 1.25 i s used, the safe b r a k i n g d i s ­tance on wet l e v e l s u r f a c e s a t 40 mph s h o u l d n o t exceed 113 f t . I f the 43 s u r f a c e s are e v a l u a t e d on t h i s b a s i s , then a l l bu t t h r ee are found t o be w i t h i n t h i s l i m i t . The

t h r e e l o c a t i o n s n o t m e e t i n g AASHO requirements are p r o j e c t s 3-6-31-123, 3 - 6 - 3 0 - 1 3 2 , and C l e v e l a n d Coun ty US 2 9 . Two o f t h e p r o j e c t s a r e s a n d - a s p h a l t pavements wh ich con­t a i n e d very f i n e s i l i c e o u s sand and an excess o f a s p h a l t w h i c h , under heavy t r a f f i c , was worked t o the s u r f a c e r e s u l t i n g i n a s l i g h t l y g l a z e d s u r f a c e t e x t u r e and v o i d s o v e r f i l l e d w i t h a spha l t . These sur­faces contained aggregates w i t h h igh ab ras ion l o s s . I n f a c t , s ince p r o ­j e c t 3-6-31-123 was t e s t e d , i t has been r e s u r f a c e d w i t h a n o n - s k i d pavement. The p r o j e c t i n Cleve land County on US 29 showed a s l i g h t ex­cess o f a s p h a l t . P o l i s h i n g o f the

49

l i m e s t o n e aggregate which was used i n t h i s mix may be a c o n t r i b u t i n g f a c t o r .

W i t h f i n e - t e x t u r e d s u r f a c e s such as sand a s p h a l t , the aggregate par ­t i c l e shape, g rada t ion and percentage o f a spha l t i s very i m p o r t a n t f rom a s k i d d i n g s t a n d p o i n t . An i m p o r t a n t f a c t o r i n s u r f a c e s h a v i n g h i g h co­e f f i c i e n t o f f r i c t i o n i s the presence o f g r i t t y p a r t i c l e s which g i v e the s u r f a c e a "sand p a p e r " t e x t u r e .

P r o j e c t 4211 shown i n T a b l e 2 c o n s i s t e d o f g r a n i t e s c r e e n i n g s graded f rom coarse t o f i n e w i t h 100 percent pass ing the No. 4 s i eve and 2 t o 4 pe rcen t pas s ing the No. 200 s i e v e . The optimum pe rcen tage o f a s p h a l t by the Hubbard -F ie ld method was 7.2S percent . E igh t and f i f t e e n one-hundredths pe rcen t a s p h a l t was e x t r a c t e d f rom cores taken f rom the pavement m p l a c e . I t i s b e l i e v e d t h a t the s u r f a c e v o i d s were o v e r ­f i l l e d w i t h a s p h a l t which produced a s l i p p e r y s u r f a c e as shown by the s k i d res i s t ance t e s t .

A f a i r l y d e f i n i t e i n c r e a s e i n the c o e f f i c i e n t o f f r i c t i o n was ob­se rved w i t h a decrease i n temper­a t u r e . The heat o f the t i r e which developed a t the h ighe r speeds when s l i d i n g s t r a i g h t ahead was l a r g e l y r e s p o n s i b l e f o r c o n s i s t e n t l y lower c o e f f i c i e n t s observed i n t h i s form o f s k i d d i n g as shown on P r o j e c t s 4211 and 3 - 9 - 3 2 - 2 0 3 . On low s t a ­b i l i t y s and-aspha l t pavements when t h e a i r t e m p e r a t u r e i s h i g h , t h e c o e f f i c i e n t s are c o n s i s t e n t l y h i g h . I n t h i s case, the wheels plow i n t o the h o t pavement as the s k i d p r o ­g r e s s e s . T h i s i s e s p e c i a l l y t r u e a t 40 mph.

INFLUENCE OF BRAKING STOESSES

Upon f a i l u r e , a b i t uminous m i x ­tu re w i l l be squeezed from under the' t i r e towards each side o f the l o n g i ­t u d i n a l lane f o l l o w e d by the wheel . I n a c t u a l s e r v i c e , however, i n ad­d i t i o n to the tendency to be squeezed from under the wheel at r i g h t angles

t o the d i r e c t i o n o f t r a v e l , b i t u m i ­nous pavements are subjec ted t o brak­i n g and a c c e l e r a t i o n s t resses . These l a t t e r f o r c e s are u s u a l l y a p p l i e d i n t h e d i r e c t i o n o f t r a v e l , and t h e y , t h e r e f o r e , a t t e m p t t o shove the pavement e i t h e r ahead o f ( f o r b r a k i n g ) , o r beh ind ( f o r a c c e l e r ­a t i o n ) the" w h e e l . S ince t hey may o f t e n p rov ide the most c r i t i c a l con­d i t i o n s o f d e s i g n f o r b i t u m i n o u s pavements c a r r y i n g moving loads , the i n f l u e n c e o f these b r a k i n g and ac­c e l e r a t i o n s t r e s s e s must be c o n ­s i d e r e d i n a q u a n t i t i v e manner, i f p o s s i b l e . From i t s very n a t u r e , i t i s p r o b a b l e t h a t b r a k i n g s t r e s s e s are g e n e r a l l y more severe than ac­c e l e r a t i o n s t resses .

The s k i d r e s i s t a n c e f a c t o r s o f r u b b e r t i r e s on s a n d - a s p h a l t and b i t u m i n o u s c o n c r e t e road s u r f a c e s are very complex, and are i n f l u e n c e d by many v a r i a b l e s and t e s t cond i t i ons . W h i l e an e f f o r t has been made i n t h i s i n v e s t i g a t i o n t o examine the e f f e c t o f some o f t he v a r i a b l e s , i t i s o u r d e s i r e t o e x p l o r e t h e e f f e c t o f age and t r a f f i c c o n d i t i o n s on these same types o f b i t u m i n o u s pavements over a p e r i o d o f a number o f years . The t e s t r e s u l t s t o date have r e v e a l e d the e f f e c t on s k i d r e s i s t a n c e o f some o f the v a r i a b l e s w h i c h have been summarized m the d i scuss ion which f o l l o w s , to i n c l u d e :

1. E f f e c t o f speed. 2 . E f f e c t o f wet versus d r y sur ­

face c o n d i t i o n . 3. E f f e c t on f r i c t i o n values o f

low s t a b i l i t y sand-asphalt mixes and s t a b l e b i t u m i n o u s c o n c r e t e m i x e s .

4. Seasonal e f f e c t on v a r i o u s su r f aces .

5. F r i c t i o n va lues as a f f e c t e d by bitumen conten t .

The two v a r i a b l e s , e f f e c t o f speed and wet versus dry sur face c o n d i t i o n , l i s t e d as I tems 1 and 2 above, and f r i c t i o n va lues as a f f e c t e d by b i ­tumen c o n t e n t , l i s t e d as I t em 5 are i m p o r t a n t c o n s i d e r a t i o n s on wh ich some work i s repor ted here.

The l o c k e d wheel b r a k i n g t e s t s

50

c l o s e l y s i m u l a t e a c t u a l v e h i c l e o p e r a t i o n and p r o v i d e an a c c u r a t e measure o f the road and t i r e f r i c ­t i o n deve loped by a passenger car when making an emergency s t o p f o r s i m i l a r road and new t i r e c o n d i t i o n s .

I n t e s t s w i t h the same car w i t h adequate brakes p r o p e r l y a d j u s t e d , c o n s i s t e n t r e s u l t s are o b t a i n e d on any g iven s u r f a c e f o r the same t e s t c o n d i t i o n s .

The c o e f f i c i e n t o f f r i c t i o n be­tween pavements and t i r e i s u s u a l l y 0 . 8 . T h i s va lue tends t o decrease as v e h i c l e speed i n c r e a s e s . For s a f e t y reasons , i t i s d e s i r a b l e t o design and cons t ruc t b i tuminouspave-ments having a very h igh c o e f f i c i e n t o f f r i c t i o n between pavement and t i r e .

ROAD ROUGHNESS

Road roughness measurements on the two types o f Bituminous pavement, sand-asphal t and a s p h a l t i c concre te pavement, are i n c l u d e d i n t h i s r e ­p o r t . As p r e v i o u s l y s t a t e d , the purpose o f the s tudy was t o o b t a i n data on r i d i n g q u a l i t i e s o f r e c e n t l y c o n s t r u c t e d b i t u m i n o u s pavements , a l s o s k i d r e s i s t a n c e t h a t c o u l d be used i n e v a l u a t i n g d e s i g n and mix p o l i c i e s and c o n s t r u c t i o n p r a c t i c e s .

A 'Doad Roughness I n d i c a t o r " de­signed by the Bureau o f P u b l i c Roads and cons t ruc ted by the Nor th C a r o l i n a S ta t e Highway and P u b l i c Works Com­m i s s i o n was used f o r these t e s t s . T h i s m a c h i n e was d e s c r i b e d i n a paper p resen ted t o the Highway Re­search Board ( 3 ) i n 1940 and l a t e r publ i shed i n Pub l i c Roads ( 4 ) . This machine measures the i t t e g u l a r i t i e s i n the road sur face which are t r a n s ­m i t t e d t h r o u g h a s t a n d a r d t i r e t o the axle o f the wheel . The v e r t i c a l movements o f t he a x l e are t r a n s ­m i t t e d by a w i r e cable t o a doub le -a c t i n g b a l l c l u t c h i n t e g r a t o r which i n t u r n t r a n s m i t s the accumula ted v e r t i c a l movements i n inches t o an e l e c t r i c counter mounted on a board i n the tow ca r . A s i m i l a r e l e c t r i c

coun te r r eco rds the r e v o l u t i o n s o f the t r a i l e r wheel and thus p rov ides an accu ra t e and dependable measure o f the t r a v e l d i s t a n c e . The rough­ness t e s t s have been s t a n d a r d i z e d a t a speed o f 20 mph, and the measure­ments are r eco rded on a da ta sheet by an o b s e r v e r f o r each h a l f m i l e and a t the end o f the p r o j e c t . The da ta are summarized by e x p r e s s i n g the roughness o f each s e c t i o n o f r o a d i n t e rms i f a s t a n d a r d u n i t known as Roughness Index ( R I ) , which i s the roughness i n inches o f v e r ­t i c a l movement per m i l e .

B e f o r e the t e s t s were s t a r t e d , the speedometer o f the t o w i n g v e ­h i c l e was c a l i b r a t e d over two meas­ured m i l e s t o de te rmine the number o f wheel r e v o l u t i o n s i n a measured m i l e .

A f u r t h e r t e s t t o de te rmine the accuracy o f the d i s t a n c e s measured by the machine c o n s i s t e d o f r epea t t e s t s made over a f i v e - m i l e s e c t i o n

na «7 Miles fitprtsnm/

F i g u r e 3. D i s t r i b u t i o n o f S u r f a c e -r o u g h n e s s M e a s u r e m e n t s f o r Sand A s p h a l t p a v e m e n t f o r t h e P r i m a r y

and Secondary Road System

51

o f new cement c o n c r e t e pavement . A l l t e a t s were run a t 20 mph i n

the d i r e c t i o n o f t r a f f i c w i t h the t r a i l e r wheel i n the c e n t e r o f the wheel t r a c k s .

The measurements inc luded i n t h i s r e p o r t were s t a r t e d i n January, 1950, and c o m p l e t e d i n December, 1950 . O n l y one d r i v e r was used on t h i s work and a l l readings were recorded by one man. Other o l d e r p r o j e c t s w h i c h had s i m i l a r d e s i g n and mix were t e s t e d f o r c o m p a r i s o n w i t h those r e c e n t l y cons t ruc t ed .

The t e s t data i n c l u d i n g the aver­age 24-hour t r a f f i c count have been t abu la t ed f o r the two types o f pave­m e n t . The v a l u e s f o r r o u g h n e s s measurements are shown i n inches per m i l e f o r o n e - h a l f m i l e i n t e r v a l s . I n t a b u l a t i n g the maximum and m i n i ­mum v a l u e s , o n l y those f o r o n e - h a l f m i l e were used. A summary o f t e s t r e s u l t s w i t h maximum, minimum, and ave rage v a l u e s f o r t he two t y p e s o f s u r f a c e t e s t e d was p r epa red t o f a c i l i t a t e compar i son o f s u r f a c e s by type .

The r e s u l t s o f t e s t s on 46 sand a spha l t p r o j e c t s are shown i n Table 3. The m a j o r i t y o f these p r o j e c t s were c o n s t r u c t e d i n 1949 and 1950. Two p r o j e c t s were c o n s t r u c t e d i n 1948. T r a f f i c on these roads v a r i e s f r o m 100 v e h i c l e s per day t o 1400 v e h i c l e s per day and averages 465 f o r a l l sand asphal t t e s t e d .

Roughness measurements f o r sand a s p h a l t averages 7 8 . 6 w i t h a max­imum o f 176 and a minimum o f 54 m . per m i .

The r e s u l t s o f t e s t s on 32 b i ­tuminous concre te pavement p r o j e c t s which t o t a l 138.5 m i . are shown m Table 4 . F i f t e e n o f these p r o j e c t s were c o n s t r u c t e d i n 1949, f i f t e e n m 1950, and one i n 1948. T r a f f i c on these roads v a r i e s f r o m 210 ve­h i c l e s per day t o 6000 v e h i c l e s per day and averages 1895 f o r a l l b i t u ­minous concrete t e s t ed .

Roughness measurements f o r b i t u ­minous c o n c r e t e pavement averages 8 2 . 4 w i t h a maximum o f 117 and a

minimum o f 57 i n . per m i . I t i s i n t e r e s t i n g t o n o t e t h e

range between maximum and minimum o f t h e two s u r f a c e t y p e s . Sand a s p h a l t has t h e l o w e s t a v e r a g e roughness ye t has the h i g h e s t range (122 i n . per m i . ) between maximum and minimum.

The roughness measurements o f each type o f s u r f a c e are a f f e c t e d by many f a c t o r s . Common t o b o t h t y p e s o f s u r f a c e s are the e f f e c t s o f subgrade or base -support , work­m a n s h i p , age, amount o f t r a f f i c , mix d e s i g n , and t r a f f i c d e n s i t i e s w h i c h a re i n f l u e n c e d by the c h a r ­a c t e r i s t i c s o f t he m a t e r i a l s and t r a f f i c ' c o n d i t i o n s .

Workmanship d e t e r m i n e s whe the r a s u r f a c e w i l l be c o n s t r u c t e d t o the s t a n d a r d s p e c i f i e d i n the mix des ign as necessary f o r smooth r i d ­i n g s u r f a c e . A p e r f e c t example o f t h i s i s p r o j e c t 3 - 7 - 2 8 - 1 5 . T h i s p r o j e c t had a s t a b i l i t y o f 800 l b s . and a d e n s i t y o f 2 .12 a t the t ime i t was c o n s t r u c t e d , ye t the t r a f f i c

F i g u r e 4 . D i s t r i b u t i o n o f S u r f a e e -r on ghnesB M e a s u r e m e n t s f o r B i t a a -i n c u s C o n c r e t e P a v e a e n t f o r t h e P r i n a r y and S e c o n d a r y Road S y s t e n

52 TABLE 3

ROAD ROUGHNESS AND STABILITIES

SAND ASPHALT PAVEMENTS Avg.

Date Length Roughness Measurements 24-Hr Lbs. Pro jec t Comp. Tested Avg. Max Min T r a f f i c Count S t a b i l i t y Density

1201 1949 5.0 73.7 81 0 68 0 180 320 2 01 1346 1950 4.0 73.0 80 0 67.0 160 597 2 10 1-7-6-85 1949 3.0 74 6 81.0 70 0 200 1225 2.14 1-7-6-19 1949 2.0 85.1 99.0 75.0 225 400 2 09 2-9-16-201 1949 7.0 66.0 73.0 60.0 100 275 2 75 2-556 1949 4.0 82.7 88.0 78.0 190 263 2 63 2-7-24- 16 1949 6.0 73.0 78.0 69.0 500 940 2.05 2-9-24-202 1949 4.0 80.6 96 0 74.0 360 1080 2.07 2-9-18-201 1949 3.0 69.0 73 0 61.0 210 1350 2 15 2-102 1950 5 0 67.7 75.0 63 0 450 260 1 80 2-7-23-3 1949 3.0 66 7 73.0 60 0 210 850 2 00 1056 1949 4.0 72.9 79 0 69 0 1070 655 2.10 2-115 1950 6.0 57.9 62 0 54.0 300 160 1.73 2551 1949 6.0 74.2 79.0 70.0 500 685 2. 10 2-7-21-19 1950 5.0 85.0 97.0 78.0 190 640 2.11 2-9-21-201 1950 3 0 84 7 98.0 74 0 200 720 2 12 2-9-17-201 1949 4 0 83.6 91.0 77 0 350 210 1.98 2566 1950 4.0 71.8 ' 76 0 68.0 1150 340 2.01 2-402 1950 4 0 74 0 85.0 63 0 360 480 1.99 3-7-28-15 1949 3.0 142.7 176.0 114 0 330 800 2 12 3-7-28-16 1949 3 0 114.4 132 0 97 0 150 850 2.13 3-7-28-09 1949 3.0 115.9 124 0 107 0 180 940 2.15 3-6-31-123 1949 7 0 92.2 113 0 83.0 1400 380 1.91 3-7-30-19 1949 1 0 78 3 86 0 71.0 225 300 2.09 3-7-30-20 1949 3 0 87 6 97.0 77.0 130 340 2 09 3-7-30-21 1949 4.0 92 0 108.0 75.0 1150 480 2.13 3-7-30-201 1949 3.5 83 7 89 0 75 0 120 500 2 13 3-6-30-132 1949 2 0 78.0 85.0 71.0 400 460 2.11 3393 1949 6.0 79.0 97 0 70 0 180 290 1 99 3-7-30-22 1949 1.0 78.0 81 0 75 0 180 310 2 01 3-7-29-08 1949 6.0 66.0 77 0 57.0 400 1140 2 20 3262 1949 4.0 63.0 70 0 56 0 500 600 2 04 3-202 1949 3.0 63.4 74.0 56.0 180 127 1.27 3-7-29-17 1949 3 0 72.0 78.0 66.0 200 930 2 07 3-7-29-13 1949 4 0 74.0 79.0 67.0 200 500 2.05 3-7-29-15 1949 3.0 75.0 78 0 68.0 125 580 2 07 3334 1949 0.8 95 5 108 0 83.0 560 60C 2 04 4-7-38-42 1949 5.0 64.9 73.0 58.0 210 1610 2.18 4-402 1949 2.0 100 3 106.0 97.0 250 390 2 01 4-7-40-17 1948 3 0 98.3 102.0 94 0 310 500 2.14 4-7-38-40 1949 1.0 90 8 93 0 90 0 180 1560 2 16 4-7-39-16 1949 2.0 85.8 91 0 81.0 210 409 2 01 4-7-39-18 1949 2 0 81.0 86.0 79 0 440 500 2 05 3864 1948 11.0 81 1 88 0 76 0 1250 2060 2.38 6-9-57-203 1949 0.37 94 2 101.0 85.0 150 1120 2 23 6-9-57-205 1950 5.0 64 7 69 0 61.0 1250 1120 2.23 46 Projec ts 173.67

Weighted Avg 78 6267 Roughness Weighted Avg 670 565 S t a b i l i t y Weighted Avg. 464 809 T r a f f i c Count Weighted Avg. 2 10333 Density

TABLE 4 53

ROAD ROUGHNESS AND STABILITIES

BITUMINOUS CCNCBETE PAVEMENTS Avg.

Date Length Roughness Measurements 24-Hr. Lbs. P ro jec t Comp. Tested Avg. Max Min T r a f f i c Count S t a b i l i t y Density

1706 1949 6.0 73.5 79.0 66.0 2250 920 2.08 1384 1949 6 0 63 2 73.0 62.0 1100 1300 2.29 3S30 1949 1.0 100.8 106.0 95.0 6000 4500 2.28 3-6-29-124 1950 6 0 73 6 93.0 58.0 2200 1180 2.23 4789 1949 1.0 72.0 77.0 68 0 1000 2050 2.18 4771 1949 2 5 74.5 81 0 69 0 3000 1510 2.04 4-6-38-166 1948 4.0 86.6 92.0 72.0 850 1835 2.20 2830 1949 1 0 79.5 82.0 78 0 2400 1940 2 30 4-6-39-123 1949 8.0 86 4 94.0 77.0 2200 1650 2 21 1614 1950 5.0 71.5 66.0 79.0 3800 2050 2.24 4772 1950 8.0 68.6 75.0 65.0 1800 1220 2.20 2851 1950 4.0 72.0 79.0 65.0 2500 1940 2.30 4722 1950 4.0 75.3 91 0 71 0 2450 4360 2.21 4330 1949 6.0 84 6 91.0 80.0 2270 2870 2.20 5-6-45-128 1949 3 0 85.8 90.0 82.0 2400 2740 2.19 3391 1950 5 0 60.8 66.0 57.0 1520 1980 2.29 5816 1950 4.0 73 6 79.0 69.0 3410 2980 2.31 6-7-56-30 1950 4.0 102.5 117.0 92.0 210 705 2 03 6-6-54-119 1950 5.0 69.6 76.0 63.0 1590 1300 2.19 7-6-61-176 1950 5.0 104.1 109.0 97.0 1000 720 2.18 6643 1950 5.0 85.0 89.0 81 0 1480 2200 2.26 6675 1950 4.0 84.0 91.0 81.0 1070 2700 2.32 7434 1949 4.0 99.0 105 0 94.0 1750 3100 2.24 7453 1949 6.0 101.4 109.0 95.0 2900 3500 2.25 8822 1950 4.0 90.0 98.0 81.0 2400 1420 2.22 9-9-80-201 1950 3.0 84.2 91.0 81.0 250 980 2.16 8444 1949 6 0 92 2 99.0 83.0 1910 1820 2.21 9514 1949 5 0 97.6 109.0 87.0 1380 4300 2.38 8361 1949 3.0 93.6 100.0 86.0 1500 4360 2.24 8280 1949 5.0 81.8 89 0 78.0 1000 1640 2.26 10-6-96-141 1950 5.0 86.1 92 0 81 0 1600 2460 2.25

31 Projects •

Weighted Average 82.42 Roughness 1895.60 T r a f f i c Count

Regul ar Average 2200.97 2.2239

Grand Tota l Weighted Avg - Roughness 80.31 Grand Total Weighted Avg - T r a f f i c Count 1,099.60 Grand Tota l Regul ar Avg. - Lbs S t a b i l i t y 1,286.70 Grand Total Regul ar Avg - Density 2.1525

Total Miles Tested - 312.00

31 Projects o f B i t . Concrete 138.5 Miles 46 Projec ts of Sand-Asphalt 173.5 Miles

TOTAL 312 0 Miles

54

count I S o n l y 330 v e h i c l e s per day. The maximum roughness i s 176 i n . per m i . , minimum 114 and weighted average o f 142 .7 . The s k i d r e s i s t a n c e co­e f f i c i e n t a t 40 mph when wet i s 0.66 or a s t o p p i n g d i s t a n c e o f 81.2 f t .

There i s a p r o g r e s s i v e , a l though s low, d i s t o r t i o n o f a pavement su r ­face w i t h age. The amount and weight o f t r a f f i c causes a gradual s e t t l i n g or breaking o f the s u r f a c e .

When t e s t i n g pavement s u r f a c e s f o r r i d i n g q u a l i t i e s , the p rob lem o f e v a l u a t i n g the road roughness measurements presents i t s e l f .

C e r t a i n ranges o f roughness were a r b i t r a r i l y se lec ted t o descr ibe the r i d i n g q u a l i t i e s o f a s u r f a c e as e x c e l l e n t , good, f a i r , or rough as shown i n Table 5.

TABLE 5

SUGGESTED STANDARDS FOR EVALUATING PAVEMENT SURFACES

Roughness Measurements ( I n . Per M i . )

Below 75 100 Above

- 75 - - 100 - 125 . - 125

R i d i n g Q u a l i t i e s

E x c e l l e n t Good Fa i r Rough

I t i s r e a l i z e d t h a t t h i s c l a s s i ­f i c a t i o n I S s u b j e c t t o debate and t h a t the type o f i r r e g u l a r i t y i n the s u r f a c e and the speed o f t he c a r have a v e r y d e c i d e d e f f e c t on the r i d i n g q u a l i t i e s o f any pavement.

SUMMARY AND CONCLUSIONS

The more i m p o r t a n t r e s u l t s and conc lus ions i n the c u r r e n t i n v e s t i ­g a t i o n o f s k i d r e s i s t a n c e and road roughness i n N o r t h C a r o l i n a may be summarized as f o l l o w s :

SKID RESULTS

Based upon more than 1500 meas­urements o f f o r w a r d s k i d d i n g d i s ­

tances on 43 pavement sur faces under c o n d i t i o n s o f t e s t s p r e v i o u s l y de­s c r i b e d , the more i m p o r t a n t r e s u l t s have been summarized as f o l l o w s :

1 . On r e l a t i v e l y smooth l e v e l pavement s , t he s topping d i s t a n c e me thod i s an e x c e l l e n t means o f d e t e r m i n i n g the average s k i d r e s i s ­t a n c e . The equipment i n v o l v e d i s r e l a t i v e l y s imple and i n e x p e n s i v e . Tes ts can be per formed and checked i n a shor t t ime .

2 . A t 40 mph, the s t o p p i n g d i s ­tance v a r i e d f r o m 6 0 . 1 t o 88.3 f t . on a d r y s u r f a c e and f r o m 7 1 . 4 t o 127.7 f t . on a wet s u r f a c e . Cor ­responding c o e f f i c i e n t s o f f r i c t i o n ranged f r o m 0 .89 t o 0 .60 on a d r y and f rom 0.75 to 0.42 on a wet sur ­f a c e . These data i n d i c a t e c l e a r l y t h a t s t o p p i n g d i s t a n c e s a t 40 mph are c r i t i c a l onl y on the wet su r f aces . I t was considered unsafe t o conduct t e s t s on wet s u r f a c e s a t a speed grea ter than 40 mph.

3. The data can be used t o es­t a b l i s h p o l i c i e s concern ing the de­s i g n , c o n s t r u c t i o n , and maintenance o f s u r f a c e s w i t h good n o n - s k i d c h a r a c t e r i s t i c s . The data ob ta ined check reasonably c lose w i t h t h a t o f previous i n v e s t i g a t o r s . (See T a b l e l )

4 . The r e s u l t s o b t a i n e d a t the three speeds can be used to p r e d i c t s t o p p i n g d i s t ances a t o the r speeds. (See Table 1)

5. A l l o f the pavements t e s t e d i n a d ry c o n d i t i o n were found to have s a t i s f a c t o r y r e s i s t a n c e t o s k i d d i n g f o r speeds o f 40 mph or l e s s .

6. For ty o f the f o r t y - t h r e e sur­faces t e s t ed wet were cons idered t o have s a t i s f a c t o r y n o n - s k i d cha rac ­t e r i s t i c s . One o f the t h r e e s u r ­faces ( P r o j e c t 3-6-31-123) found t o be unsafe from a s k i d d i n g s t andpo in t has been r e su r f aced .

7. For the su r faces t e s t e d wet , the two groups l i s t e d i n accordance w i t h the s t o p p i n g d i s t a n c e f rom the lowest average to the h ighes t average are as f o l l o w s :

Sand asphal t a t 40 MPH Wet Average 91.32 Feet.

55

A s p h a l t Concre te a t 40 MPH Wet Average 92.94 Feet.

I t s h o u l d be p o i n t e d o u t t h a t t h i s r a t i n g a p p l i e s o n l y t o t h e s u r f a c e s t e s t e d , and s i n c e o n l y a few s u r f a c e s are i n c l u d e d i n t h e a s p h a l t i c conc re t e group, too much emphas is s h o u l d n o t be p l a c e d on t h i s comparison.

8. S k i d d i n g r e s i s t a n c e v a r i e s w i t h the t e x t u r e and compos i t i on o f the pavement s u r f a c e . Those s u r ­faces hav ing a harsh , g r i t t y , "sand p a p e r " t e x t u r e were found t o have s h o r t s topping d i s t ance .

9. With sand a spha l t c o n t a i n i n g s c r e e n i n g s , the s t o p p i n g d i s t a n c e a t 40 mph on a wet s u r f a c e v a r i e d d i r e c t l y as the abrasion loss o f the s t o n e s c r e e n i n g s , o t h e r f a c t o r s b e i n g e q u a l . I n o t h e r words , l o n g s t o p p i n g d i s t ances were measured on the sand asphal t surfaces c o n t a i n i n g 100 p e r c e n t s tone s c r e e n i n g s w i t h h igh abrasion loss and shor t e r s top­p ing dis tances were encountered w i t h s u r f a c e s c o n t a i n i n g low a b r a s i o n stone sc reen ings . T y p i c a l examples are Vance County US 1 w i t h a wear t e s t i f 67 and s k i d d i s t ance o f 111.0 f t ; Rowan County US 29 w i t h a wear t e s t o f 36 and a s k i d d i s ­tance o f 95.6 f t .

10,. A s l i g h t excess i n the pe r ­cent a s p h a l t w i l l have a more p r o ­nounced e f f e c t i n r educ ing the f r i c ­t i o n values than the p a r t i c l e shape o f the aggregate. Excessive asphal t tends t o decrease the s t a b i l i t y o f the pavement.

1 1 . Higher f r i c t i o n va lues have been observed i n the W i n t e r t e s t s than i n the Summer t e s t s . T h i s i s e s p e c i a l l y t r u e where the pavements have low s t a b i l i t i e s .

ROUGHNESS RESULTS

The more i m p o r t a n t r e s u l t s o f the road roughness measurements on the two types o f pavement (77 p r o ­j e c t s ) t o t a l i n g 312 m i . and r e p r e ­s e n t i n g a p p r o x i m a t e l y 800 m i . are summarized as f o l l o w s :

1. The Road Roughness I n d i c a t o r i s a c o n v e n i e n t , s a t i s f a c t o r y , and r a p i d means o f e v a l u a t i n g r i d i n g q u a l i t i e s o f p a v e m e n t s u r f a c e .

2. Test r e s u l t s can be recorded i n a f o r m c o n v e n i e n t f o r s t u d y or comparison o f two or more p r o j e c t s .

3 . Repeat t e s t s w i t h the Road Roughness I n d i c a t o r g i v e r e s u l t s t h a t check c l o s e l y c o n s i d e r i n g the i n a b i l i t y o f the d r i v e r t o f o l l o w i n e x a c t l y the same path on each r u n .

4 . R i d i n g q u a l i t i e s o f a l l pave­ments and, t h e r e f o r e , road roughness measurements are a f f t i c t e d by such f a c t o r s as workmanship d u r i n g con­s t r u c t i o n , pavement age, amount and type o f t r a f f i c .

5. R i d i n g q u a l i t i e s o f any r e l a t i v e l y t h i n su r f ace course such as sand a spha l t and b i tuminous con­c re te are i n f l u e n c e d by the s t a b i l i t y o f the base and sur face course.

6. T e x t u r e o f b i t u m i n o u s s u r ­faces has a b e a r i n g on road r o u g h ­ness measurements.

7 . Sand a s p h a l t s u r f a c e s were found t o have the best r i d i n g q u a l ­i t i e s .

8. Bi tuminous concre te s u r f a c e s were s l i g h t l y r o u g h e r t h a n sand a spha l t .

ACKNOWLEDGMENTS

The a u t h o r w i shes t o a c k n o w l e d g e , w i t h s i n c e r e a p p r e c i a t i o n , t h e h e l p g i v e n by a l l those who have a s s i s t e d i n t h i s i n v e s t i g a t i o n . S p e c i a l c r e d i t s h o u l d be g i v e n t o M e s s r s H . C. G i l l i s , G e o r g e M o o r e , a n d L u t h e r T y s o n o f t h e H i g h w a y E q u i p ­m e n t D i v i s i o n f o r t h e i r w o r k i n b u i l d i n g t h e ' l i o a d - Rou gh n e s s I n d i ­c a t o r " and c e r t a i n o t h e r i t e m s o f t e s t e q u i p m e n t u s e d i n b o t h t h e roughness and s k i d r e s i s t a n c e t e s t s .

S p e c i a l t h a n k s a r e e x t e n d e d t o M r . C E. P r o u d l e y , C h i e f M a t e r i a l s and T e s t E n g i n e e r , f o r t h e h e l p f u l c r i t i c i s m and s u g g e s t i o n s i n t h e p r e p a r a t i o n o f t h i s r e p u r t . O t h e r s p e c i a l commendat ion s h o u l d be g i v e n

56

t o M e s s r s . C . R . P e n n y a n d J . T . N o r t o n f o r t h e i r s k i l l f u l h a n d l i n g o f the e q u i p i i e n t d u r i n g t e s t s .

v ' REFERENCES

1. Moyer, R. A . , 'Wotor V e h i c l e O p e r a t i n g C o s t s , Road Roughness and S l i p p e r i n e s s o f V a r i o u s P o r t l a n d Cement C o n c r e t e S u r f a c e s , " Proceed-ings, Highway R e s e a r c h B o a r d , V o l . 22, pp. 13-52 .J 1942.

2 . S h e l b u r n e , T . E . and Sheppe,

R. L . , ' S k i d R e s i s t a n c e Measurements o f V i r g i n i a P a v e m e n t s , " H i g h w a y R e s e a r c h B o a r d , Research Report No. 5-B, 1 9 4 8 .

3. Buchanan, J . A . , and C a t u d a l , A. L . , ' S t a n d a r d i z a h l e Equipment for E v a l u a t i n g Road S u r f a c e R o u g h n e s s , " Proceedings, Highway R e s e a r c h Board , V o l . 20, pp. 621-634, 1940.

4. Public Roads. V o l . 2 1 , No. 12, pp. 227-234, Feb. 1941.

5. American A s s o c i a t i o n of S t a t e H i g h w a y O f f i c i a l s . "A P o l i c y on S i g h t D i s t a n c e for H i g h w a y s , " 1940.